Wireless communication systems, i.e. systems that provide communication services to wireless communication devices such as mobile phones, smartphones (often denoted by UE that is short for user equipment) as well as machine-type communication (MTC) devices, have evolved during the last decade into systems that must utilize the radio spectrum and other system resources in the most efficient manner possible. A reason for this is the ever increasing demand for high speed data communication capabilities in terms of, e.g., bitrate and to provide these capabilities at any given time, at any geographical location and also in scenarios where the wireless communication device is moving at a high speed, e.g., on board a high speed train (HST). To meet this demand much work is being done within the third generation partnership project (3GPP) for enhancing performance in high speed train environments. The justification is that there are railways such as Japan Tohoku Shinkansen (running at 320 km/h), German ICE (330 km/h), AGV Italo (400 km/h), and Shanghai Maglev (430 km/h) at which vehicles travel at greater speed than 300 km/h and where there is demand for a large number of simultaneous users using mobile services while being on-board such a HST.
Given the fact that mobility is one of the corner stones of the 3GPP system, mobility management is conventionally carried out individually, e.g. base stations such as evolved NodeB's (eNodeB) in a 3GPP long term evolution (LTE) system configure radio frequency (RF) signal measurement events and provide configuration information to wireless communication devices via radio resource control (RRC) signalling. Having received such configuration information, a wireless communication device measures requested parameters such as reference signal received power (RSRP) and/or reference signal received quality (RSRQ) and reports the results back to base stations, e.g. eNodeB, periodically or when specific event criterions are met. A handover decision is made when a target cell is more suitable than a serving cell.
As FIG. 1 illustrates, a handover procedure in 3GPP wireless communication system typically involves a signalling sequence between a wireless communication device (denoted UE), a source eNodeB, a target eNodeB, a mobility management entity (MME) and a serving gateway (SGW). In summary, a conventional handover procedure as shown in FIG. 1 comprises, from the point of view of a source cell (i.e a source eNodeB) obtaining of RF signal measurements from the wireless communication device and, after having analysed the measurements, informing a target eNodeB to take over control of the wireless communication device. Having taken over communication with the wireless communication device via a random access (RA) procedure, the target eNodeB controls a transfer of radio bearers between the wireless communication device and the SGW.
In situations where many wireless communication devices are moving together, e.g. on-board a high speed train carrying a large number of passengers, such conventional mobile management method may become problematic due to generation of a large number of handover requests during very short time periods. Examples of such situations are described in United States patent application publication 2015/0181481 and in “An Enhanced Handover Scheme for Mobile Relays in LTE-A High-Speed Rail Networks”, IEEE Transactions on Vehicular Technology V. 64 No. 2, 763 (2015).
Existing solutions, such as those cited above, are focused on mobility management with the aim of reducing the number of handover events of the group of wireless communication devices. This means a preference for larger cells and lower radio frequencies.
A drawback with such a low radio frequency scenario is that, since a group of wireless communication devices will require a relatively high total capacity in terms of, e.g., bitrate and response time, use of larger cells having lower radio frequencies will limit the high capacity requirement. It is therefore desirable to find a way to provide high capacity to groups of wireless communication devices that is not restricted to low radio frequency solutions. In other words, a solution that makes use of higher radio frequencies and therefore enables higher capacity is desirable.
However, since normally cell sizes become smaller at higher radio frequencies, handover events become ever more frequent and the degradation of performance due to handovers becomes severe. Such a drawback may be mitigated by increasing the size of a cell by configuring a few radio base station radio beam sectors to form a “supercell”, yet the handover issues still need to be handled between such “supercells”.